Optical pickup and optical disc drive device

ABSTRACT

The present invention relates to an optical pickup and an optical disc drive that records or reproduces data signals on or from optical discs different in specifications, such as recording density and thickness. One objective lens ( 17 ) is used for optical discs ( 22   a,    22   b ) of two types that differ in specifications. The optical pickup and the optical disc drive comprise two light source sections ( 24   a,    24   b ) and a lens-supporting mechanism ( 15 ). The light source sections emit two laser beams of different wavelengths, respectively. The lens-supporting mechanism ( 15 ) controls the position of the objective lens ( 17 ) in the radial direction of the discs and along the optical axis. The lens-supporting mechanism ( 15 ) has a lens-inclination adjusting mechanism ( 30 ) capable of changing an angle at which the objective lens ( 17 ) is inclined to the optical axis, substantially in proportion to a displacement of the optical disc of the first type from the reference point in the signal-recording plane of the optical disc of the first type.

TECHNICAL FIELD

[0001] The present invention relates to an optical pickup that canrecord or reproduce data signals on or from optical discs of differentspecifications. The invention also relates to an optical disc drive thatcomprises the optical pickup.

BACKGROUND ART

[0002] Optical disc drives are known, which apply a laser beam tooptical discs, i.e., disc-shaped optical recording media, thereby torecord data signals on the optical discs or reproduce data signals fromthe optical discs. Some of the optical disc drives incorporate oneoptical pickup that can record or reproduce data signals on or fromoptical discs that differ in specification such as thickness, recordingdensity and the like.

[0003]FIG. 1 shows an optical pickup configured to record data signalson optical discs of different specifications or to reproduce datasignals recorded on optical discs. As FIG. 1 depicts, an optical pickup101 has two semiconductor lasers 102 and 103, two beam splitters 104 and105, a collimator lens 106, an objective lens 107, a lens-supportingmechanism 108, a light-receiving element 109, and the like. Thesemiconductor lasers 102 and 103 are laser beam emitting elements thatemits laser beams different in wavelength. The lens-supporting mechanism108 includes an actuator that drives and displaces the objective lens107. The light-receiving element 109 receives the light beam reflectedfrom an optical disc 110.

[0004] Various methods of enhancing the density at which to record datasignals on optical discs are available. Of these methods, the mostgenerally used is to use an objective lens having a large numericalaperture (NA) and apply a light beam having a short wavelength (λ) toread data signals recorded on the optical disc. If the recording mediumused is a compact disc (CD), there is used an optical pickup thatcomprises an objective lens having a numerical aperture (NA) of 0.45 andemits a light beam having a wavelength (λ) of 780 nm. If the recordingmedium is a digital versatile disc (DVD), there is used an opticalpickup that comprises an objective lens having a numerical aperture (NA)of 0.6 and emits a light beam having a wavelength (λ) of 650 nm. Notethat the diameter of a beam spot formed on the signal-recording plane ofthe optical disc is proportional to λ/NA. Hence, the beam spot formed onany DVD has a diameter that is about 63% of the diameter of the beamspot formed on the CD.

[0005] The larger the numerical aperture (NA) of the objective lens, themore greatly the beam spot will be deformed when the optical disc isinclined to the light beam. More specifically, the beam spot will bemore deformed as the coma-aberration increases in proportion to the cubeof NA. Thus, the protective layer of the DVD is made thinner than thatof the CD so as to reduce the beam-spot deforming caused by theinclination of the optical disc to the light beam.

[0006] In order to record or reproduce data signals on or from two typesof optical discs, e.g., a CD and a DVD, by one objective lens, the lightbeam must be focused to form a beam spot of such a size as would recordor reproduce data signals as is desired. Such an objective lens isdescribed in, for example, the specification of Japanese Patent No.2559006.

[0007] The CD and the DVD are almost identical in terms of diameter andthickness. However, the protective layer of the DVD is thinner than thatof the CD, as pointed out above. The CD and the DVD inevitably differ inthe position of the signal-recording plane. Due to this difference, theCD and the DVD are regarded as a thick disc and a thin disc,respectively.

[0008] Being thin, optical discs may warp in their radial direction,from the center to the outer circumference. Consequently, so-called“plane wobbling” is likely to occur as any optical disc is rotated. Thewarping and/or plane wobbling of the optical disc results in acoma-aberration. The coma-aberration gives rise to a crosstalk, becausethe beam spot formed on the disc is deformed. The crosstalk renders thesignals read from the disc unclear. To minimize the deforming of thebeam spot, various methods can be employed. One method is to detect theinclination of the optical disc electrically and the aberration isreduced by an aberration-correcting element in accordance with the discinclination detected, as is disclosed in Jpn. Pat. Appln. Laid-OpenPublication No. 11-110802. Another method is to use a triaxial actuatorthat can move the objective lens in three directions, i.e., focusingdirection, tracking direction and tilting direction, as is disclosed inJpn. Pat. Appln. Laid-Open Publication No. 2000-36125. These methodsdescribed in the Laid-Open publications, i.e., methods of minimizing thedeforming of the beam spot formed on the optical disc, requireadditional components. The additional components render the opticalpickup complicated in structure and raise the manufacturing cost of theoptical pickup. Ultimately, they will increase the structural complexityand manufacturing cost of the optical disc drive that uses this opticalpickup.

[0009] A method of preventing the deforming of a beam spot has beenproposed. In the method, a two-axis actuator is inclined to the radialdirection of the optical disc, in proportion to the displacement of theobjective lens from a prescribed position. That is, the actuator ismoved to follow the warping of the optical disc, thereby to prevent thedeforming of the beam spot. To record or reproduce data signals on orfrom two types of optical discs that are different in thickness, bymeans of a single objective lens, it is necessary to adjust the workingdistance, i.e., the distance between the objective lens and theincidence side of the optical disc. When the same support supportseither type of an optical disc and the inclination of the objective lensis set for one type of an optical disc, however, the objective lens isinclined to the other type of an optical lens by the angle proportionalto the difference between the two types of discs in terms of workingdistance. Inevitably, the optical pickup reproduces degraded datasignals.

DISCLOSURE OF THE INVENTION

[0010] An object of this invention is to provide a novel optical pickupthat can solve the above-mentioned problems with the conventionaloptical pickup and conventional optical disc drive, and to provide anoptical disc drive that comprises this novel optical pickup.

[0011] Another object of the invention is to provide an optical pickupthat has a simple structure and can yet prevent the deforming of thebeam spot formed as a light beam is applied to an optical disc, therebyto record or reproduce data signals on or from the optical disc, withgood recording/reproducing characteristics, and to provide an opticaldisc drive that comprises this optical pickup.

[0012] According to this invention made to achieve the objects specifiedabove, there is provided an optical pickup optical pickup designed torecord or reproduce data signals on or from two types of optical discsthat differ in at least data recording density and thickness, by usingthe same objective lens. The optical pickup comprises: two light sourcesections for emitting laser beams having different wavelengths; and alens-supporting mechanism for controlling a position of the objectivelens in a radial direction of the optical discs and along an opticalaxis. The lens-supporting mechanism has a lens-inclination adjustingmechanism capable of changing an angle at which the objective lens isinclined to the optical axis, substantially in proportion to adisplacement of the optical disc of the first type. The second lightsource section for recording or reproducing data signals on or from theoptical disc of the second type is positioned with respect to theposition of the first light source section for recording or reproducingdata signals on or from the optical disc of the first type, so as tosatisfy the following condition:

120°≦θ≦240°

[0013] where θ is an angle measured counterclockwise from the firstlight source section used to record or reproduce data signals on or fromthe optical disc of the first type to the second light source sectionused to record or reproduce data signals on or from the optical disc ofthe second type, along a straight line in which the optical pickup movestoward an outer circumference of the optical disc to record or reproducedata signals on or form the optical disc. The optical pickup eliminatesthe deterioration of the beam spot, which results from the warping ofthe optical disc of the first type. Additionally, it can minimize theinfluence imposed on the optical disc of the second type by theobjective lens inclined by the lens-inclination adjusting mechanism.

[0014] The present invention also provides an optical disc driveapparatus designed to record or reproduce data signals on or from twotypes of optical discs that differ in specifications such as datarecording density, thickness and the like, by using the same objectivelens. The optical pickup comprises: two light source sections foremitting laser beams having different wavelengths, and a lens-supportingmechanism for controlling a position of the objective lens in a radialdirection of the optical discs and along an optical axis. Thelens-supporting mechanism has a lens-inclination adjusting mechanismcapable of changing an angle at which the objective lens is inclined tothe optical axis, substantially in proportion to a displacement of theoptical disc of the first type from a reference point in asignal-recording plane of the optical disc of the first type. The secondlight source section for recording or reproducing data signals on orfrom the optical disc of the second type is positioned with respect tothe position of the first light source section for recording orreproducing data signals on or from the optical disc of the first type,so as to satisfy the following condition:

120°≦θ≦240°

[0015] where θ is an angle measured counterclockwise from the firstlight source section used to record or reproduce data signals on or fromthe optical disc of the first type to the second light source sectionused to record or reproduce data signals on or from the optical disc ofthe second type, along a straight line in which the optical pickup movestoward an outer circumference of the optical disc to record or reproducedata signals on or from the optical disc. The optical disc driveapparatus can eliminate the deterioration of the beam spot, whichresults from the warping of the optical disc of the first type, therebyto optimize the data signals recorded or reproduced. Further, theapparatus can minimize the influence imposed on the optical disc of thesecond type by the objective lens inclined by the lens-inclinationadjusting mechanism.

[0016] In the optical pickup and the optical disc drive apparatus, bothaccording to this invention, two light source sections are supported onthe same support member. Therefore, the pickup and the apparatus can berendered compact as a whole.

[0017] In the optical pickup and the optical disc drive apparatus, bothaccording to this invention, the lens-inclination adjusting mechanism isconfigured to change the angle at which the objective lens is inclined,to satisfy the following condition:

1/4·δ/R≦S≦δ/R

[0018] where S is the inclination angle of the objective lens (regardedas positive if the objective lens approaches the outer circumference ofthe optical disc of the first type), δ is the displacement of theobjective lens from a reference position (regarded as positive if theobjective lens approaches the optical disc), and R is the radius of theoptical disc. Thanks to this configuration, it is possible to eliminatethe deterioration of the beam spot efficiently, which results from thewarping of the optical disc of the first type.

[0019] The optical pickup and the optical disc drive apparatus, bothaccording to this invention, further comprise a light-receiving sectionfor receiving laser beams reflected by the optical discs, and meansarranged between the light-receiving section and the objective lens.Said means combines optical paths of two types of laser beams emittedfrom the two light sources, respectively, such that the two laser beamsreflected by the optical discs are received at substantially the samepoint on the light-receiving section. Hence, the light-receiving sectionworks for two types of optical discs. This helps to reduce the number ofcomponents.

[0020] Further, the optical pickup and the optical disc drive apparatus,according to this invention, satisfy the following conditions:

−0.3≦T·ΔWD·φ+tan ⁻¹(ΔL·|β|/f)·α·cos θ≦0.3

0.2≦tan ⁻¹(ΔL·|β|/f)·α·sin θ≦0.2

[0021] where f is the focal distance of the objective lens; β is themagnification of the optical system; ΔWD is the difference between theworking distance of recording or reproducing data signals on or from theoptical disc of the first type and the working distance of recording orreproducing data signals on or from the optical disc of the second type;ø is the angle at which the optical disc of the second type is inclinedto record or reproduce the signals in optimal conditions when theincidence angle of the laser beam applied to the objective lens changesby one degree; α is the angle at which the optical disc of the secondtype is inclined to optimize the signals recorded or reproduced, whenthe incidence angle of the laser beam applied to the objective lenschanges by the one degree (regarded as positive if the disc is inclinedto eliminate the influence of inclination of the laser beam); T is theproportionality constant determined by the radius of the optical disc ofthe first type, which relates the displacement of the objective lens(regarded as positive if the objective lens approaches the optical discof the first type) from a reference position and the inclination of theobjective lens (regarded as positive if the side of the objective lenslocated at the outer circumference of the optical disc of the first typeapproaches the optical disc of the first type); ΔL is the distancebetween the first light source section used to record or reproduce datasignals on or from the optical disc of the first type and the secondlight source section used to record or reproduce data signals on or fromthe optical disc of the second type; and θ is an angle measuredcounterclockwise from the first light source section to the second lightsource section, along a straight line in which the optical pickup movestoward an outer circumference of the optical disc to record or reproducedata signals on or form the optical disc. Hence, this optical pickup caneffectively reduces the influence, such as coma-aberration, imposed onthe optical disc of the second type, too, by the objective lens inclinedby the lens-inclination adjusting mechanism.

[0022] The other objects of this invention and the other advantagesachieved by the invention will be apparent from the embodiments thatshall be described below, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a side view of a conventional optical system of theoptical pickup, depicting the basic structure of the optical pickup;

[0024]FIG. 2 is a perspective view, illustrating the outer appearance ofan optical disc drive according to the invention;

[0025]FIG. 3 is a partly exploded, perspective view of the optical discdrive, showing the basic structure thereof;

[0026]FIG. 4 is a perspective view of the biaxial actuator incorporatedin the optical pickup;

[0027]FIG. 5 is a side view of the optical system of the optical pickup,illustrating the basic structure of the optical pickup;

[0028]FIG. 6 is a diagram explaining the operation of a lens-inclinationadjusting mechanism;

[0029]FIG. 7 is another diagram explaining the operation of thelens-inclination adjusting mechanism;

[0030]FIG. 8 is a schematic diagram showing a warping optical disc;

[0031]FIG. 9 is a diagram explaining how the signal-recording plane ofthe optical disc is displaced when the optical disc warps;

[0032]FIG. 10 is a diagram representing the relation between thedisplacement of the signal-recording plane and the inclination of theobjective lens, which is observed at various positions along the radiusof the optical disc;

[0033]FIGS. 11A and 11B are diagrams showing the positions that theobjective lens takes with respect to optical discs of differentspecifications when it is moved to record or reproduce data signals onor from the optical discs;

[0034]FIG. 12A is a diagram showing how coma-aberration occurs when theobjective lens inclines to the optical disc;

[0035]FIG. 12B is a diagram explaining how coma-aberration occurs whenthe optical disc inclines to the objective lens;

[0036]FIG. 13 is a graph representing the relation between theinclination of the objective lens or optical disc and thecoma-aberration;

[0037]FIG. 14 depicts the positional relation of the two light sourcesprovided in a laser;

[0038]FIG. 15A is a graph showing the relation between thecoma-aberration and the incident angle of a laser beam applied to theobjective lens; and

[0039]FIG. 15B is a graph representing the relation between theinclination of the optical disc and the coma-aberration.

BEST MODE FOR CARRYING OUT THE INVENTION

[0040] An optical pickup according to the invention and an optical discdrive comprising the optical pickup of the invention will be described,with reference to the accompanying drawings.

[0041] The embodiments described below are an optical disc drive thatuses, as recording medium, a DVD (Digital Versatile Disc or DigitalVideo Disc, hereinafter referred to as “DVD”) or a CD (Compact Disc,hereinafter referred to as “CD”) such as CD-ROM, CD-R, CD-RW or thelike, and an optical pickup that is incorporated in this optical discdrive.

[0042] The optical disc drive according to the invention will be firstdescribed.

[0043] As FIG. 2 shows, the optical disc drive 1 has a housing 2 andvarious components and mechanisms contained in the housing 2. Thehousing 2 comprises a cover 3 and a front panel 4. The front panel 4 hasan elongate opening 4 a and an operation section 5. The opening 4 aextends horizontally. The operation section 5 has various operationbuttons.

[0044] As FIG. 3 depicts, the housing 2 contains a mechanism frame 6.The mechanism frame 6 supports various mechanisms. Among the mechanismsare a loading mechanism 8 that loads a disc tray 7 and a base unit 9that can rotate. The base unit 9 holds a disc table 11 and an opticalpickup 12. The disc table 11 can be rotated by a spindle motor 10. Theoptical pickup 12 is supported by the base unit 9, able to move in theradial direction of an optical disc that is mounted on the disc table 11and can be rotated.

[0045] As shown in FIG. 3, the optical pickup 12 comprises a movablebase 13 and some other parts arranged on the movable base 13. Moreprecisely, two bearing units 13 a and 13 b are provided at the ends ofthe movable base 13, respectively. The bearing units 13 a and 13 bsupport two guide shafts 14, respectively. The guide shafts 14 extendparallel to each other. Guided by these shafts 14, the optical pickup 12can move in the axial direction of the optical disc mounted on the disctable 11 and being spinning.

[0046] A biaxial actuator 15 is provided on the movable base 13. Theactuator 15 serves as a lens-supporting mechanism. As seen from FIG. 4,the biaxial actuator 15 comprises a fixed part 15 a and a biaxialmovable part 15 b. The fixed part 15 a is secured to the movable base13. The biaxial movable part 15 b comprises a holding member 16, anobjective lens 17, and a coil section 18. The holding member 16 iscomposed of a lens holder 16 a and a coil bobbin 16 b that are formedintegral with each other. The coil bobbin 16 b protrudes from one sideof the lens holder 16 a. The lens holder 16 a holds the objective lens17. The lens 17 is pressed into or adhered to the lens holder 16 a. Thelens 17 may be secured to the lens holder 16 by any other appropriatemethod.

[0047] The optical pickup 12 of the structure described above furthercomprises a lens-inclination adjusting mechanism 30. The operatingprinciple of the mechanism 30 will be described. Additionally, therelation between the inclination of the optical disc and the beam spotformed by the laser beam applied to the optical disc will be explained.Note that the inclination of the optical disc results from the warpingand plane wobbling of the disc.

[0048] The biaxial movable part 15 b provided in the optical pickup 12of this invention is secured and electrically connected to the fixedpart 15 b by four suspensions 19, 19, . . . . The suspensions 19 arethin wires made of electrically conductive and elastic material. Thefour suspensions 19 are arranged at different positions and havedifferent spring constants that depend on their positions as will bespecified later.

[0049] The fixed part 15 a holds a yoke 20 that has a substantiallyU-shaped cross section. The yoke 20 comprises a pair of yoke strips 20 aand 20 a that oppose each other. Two magnets 21, 21 are fixed to theopposing sides of the yoke strips 20 a and 20 a, respectively. The yokestrips 20 a and 20 a and the magnets 21, 21 constitute a magneticcircuit, jointly with a focusing coil 18 a and tracking coils 18 b, 18b.

[0050] The optical disc drive 1 according to the invention is a devicethat drives either a DVD or a CD as explained earlier. It uses one oftwo laser beams of different wavelengths, i.e., 650 nm and 780 nm, inaccordance with the type of the disc it drives. The objective lens 17that the disc drive 1 incorporates is a so-called “bifocal lens.”

[0051] In the optical disc drive according to this invention, theloading mechanism 8 pushes out the disc tray 7 from the opening 4 a madein the front panel 4 when the user operates the operation section 5provided on the front panel 4, inputting an unloading command to movethe disc tray 23 to a disc insertion/removal position outside thehousing 2. Once the disc tray 7 is so pushed out, an optical disc can beplaced in the recess 7 a made in the disc tray 7. After the optical discis placed in the recess 7 a, the user may operate the operation section5 again, inputting a loading command to move the disc tray 7 back intothe housing 2. In response to this command the loading mechanism 8 pullsthe disc tray 7 into the housing 2.

[0052] When the user operates the operation section 5, thus inputting acommand to record or reproduce a data signal on or from the optical disc(DVD) 22 a or the optical disc (CD) 22 b, the base unit 9 is rotated,moving the disc table 11 upwards in FIG. 3. As a result, the center partof the disc table 11 is inserted into the center hole of the opticaldisc. The optical disc is thereby chucked to the disc table 11. Theoptical disc chucked to the table 11, i.e., the DVD 22 a or the CD 22 b,can therefore rotate together with the disc table 11. When the disctable 11 is driven to rotate by the spindle motor 10, the DVD 22 a orthe CD 22 b is rotated together with the disc table 11. When the disctable 11 is rotated, the optical pickup 12 is moved from the innercircumference of the CD 22 b or DVD 22 a, which is spinning togetherwith the disc table 11, toward the outer circumference of the CD 22 b orDVD 22 a, while guided by the pair of guide shafts 14, 14. Meanwhile, alaser beam is applied to the signal-recording plane of the DVD 22 a orCD 22 b through the objective lens 17 held by the biaxial actuator 15. Adata signal is written on the DVD 22 a or the CD 22 b, or a data signalrecorded on the DVD 22 a or the CD 22 b is read.

[0053] The user may operate the operation section 5 provided on thefront panel 4, inputting a command to stop the reproducing or recordingof the data signal. If this is the case, the CD 22 b or the DVD 22 a isstopped. At the same time the optical pickup 12 is stopped. The opticalpickup 12 therefore stops reproducing or recording the data signal. Thebase unit 9 is rotated downwards (FIG. 3), or in the direction oppositeto the above-mentioned direction. The CD 22 b or the DVD 22 a is therebyreleased from the disc table 11, the optical disc is placed in therecess 7 a of the disc tray 7, and the base unit 9 moves to a positionbelow the disc tray 7.

[0054] The user may operate the operation section 5, inputting a commandto push the disc tray 7 out of the housing 2. In this case, the disctray 7 is pushed out from the housing 2 through the opening 4 a. Hence,the CD 22 b or the DVD 22 a can be removed from the recess 7 a. When theuser removes the CD 22 b or the DVD 22 a and operates the operationsection 5, thus inputting a command to pull the disc tray 7 into thehousing 2, the disc tray 7 is pulled into the housing 2.

[0055] The optical system 23 of the optical pickup 12, which emits alaser beam, will be described.

[0056] The optical system 23 comprises an optical base 23 a. All othercomponents of the system 23, except the objective lens 17, are securedin place on the optical base 23. As FIG. 5 depicts, the optical system23 comprises two light sources 24 a and 24 b that emit a laser beamhaving a wavelength of 650 nm and a laser beam having a wavelength of780 nm, respectively. The light source 24 a that emits a 650-nm laserbeam is provided for DVDs, whereas the light source 24 b that emits a780-nm laser beam is provided for DCs. Both light sources 24 a and 24 bare arranged on the same support member 24 c and provided in the form ofa single laser element 24. The optical system 23 comprises, besides theobjective lens 17, a diffraction element 25, a beam splitter 26, acollimator lens 27, a light-receiving element (PDIC) 28, and an opticalelement 29. The diffraction element 25 diffracts the 780-nm laser beamto be applied to the CD 22 b, thereby to form a beam spot that serves toachieve tracking servo. The beam splitter 26 allows passage of a laserbeam or reflects the same. The light-receiving element 28 receives thelaser beam reflected by the signal-recording plane of the DVD 22 a or CD22 b. The optical element 29 functions as means for combining the pathsof laser beams having different wavelengths, thereby to apply the laserbeams at the same position on the light-receiving element 28. Theoptical element performs another function; it makes the laser beamsundergo astigmatism, thereby to generate a focusing error signal.

[0057] The DVD 22 a and the CD 22 b, both used in the optical disc drive1, have essentially the same thickness. Nonetheless, they differ interms of operating point, i.e., the position where the laser beam isfocused to form a beam spot. This is because their signal-recordingplanes are different in position. More specifically, the DVD 22 a andthe CD 22 b have operating points at different distances from theincident plane of the laser beam, one distance longer than the other by0.6 mm, when both discs are placed at the same level on the disc table11. This difference of 0.6 mm results in a difference in sphericalaberration. The difference in spherical aberration must be eliminated tofocus the laser beams on both the DVD 22 a and the CD 22 b as isdesired. To this end, the objective lens 17 used in the optical discdrive 1 is a so-called “bifocal lens” as pointed out above. Although theDVD 22 a and the CD 22 b have the same thickness, they may be regardedas a thin disc and a thick disc, respectively, since theirsignal-recording planes lie at different levels.

[0058] However, the objective lens 17 cannot help to eliminate theoperating point difference of 0.6 mm though it is bifocal lens. Theobjective lens 17 is therefore driven by the biaxial actuator thatincludes the lens-supporting mechanism 15. Thus driven, the lens 17moves along its optical axis and in the focusing and tracking directionswhich are respectively parallel and perpendicular to the optical axis.Hence, the lens 17 focuses the laser beam, forming a desirable beam spoton the recording track of the DVD 22 a or the CD 22 b, thereby to reador write data reliably from or on the recording track. The objectivelens 17, i.e., the bifocal lens, is designed to have two theoreticaloperating points which are spaced in the focusing direction. The lensoperates at one of these operating points to record and reproduce datasignals on and from CDs, and at the other operating point to record andreproduce data signals on and from DVDs. To be more specific, theobjective lens 17 has the operation point at position A (hereinaftercalled “basic operating point” shown in FIG. 5) for the DVD 22 a, and atposition B (hereinafter called “basic operating point,” shown in FIG. 5)for the CD 22 b.

[0059] The biaxial actuator 15, which supports the objective lens 17 andmoves the same in both the focusing direction and the trackingdirection, comprises the lens-inclination adjusting mechanism 30. Thismechanism 30 inclines the objective lens 17 by an inclination angle S tothe radial direction of the DVD 22 a. (The angle S has a positive valuewhen the outer circumference of the DVD 22 a approaches the lens 17.)The angle S is proportional to the displacement distance δ (mm) theobjective lens 17 has moved from the basic operating point A to recordor reproduce data signals on or from the DVD 22 a.

[0060] The lens-inclination adjusting mechanism 30 is a mechanism thatutilizes the function of the biaxial actuator 15 to incline theobjective lens 17. To enable to the mechanism 30 to incline the lens 17,only some of the components of the biaxial actuator 15 have beenmechanically modified.

[0061] Namely, the lens-inclination adjusting mechanism 30 functions asis described in Jpn. Pat. Appln. Laid-Open Publication 2001-319353 filedby the applicant and also in U.S. patent application Ser. No. 09/842,868that corresponds to Publication 2001-319353. Though not detailed here,the mechanism 30 is designed by modifying the biaxial actuator 15. Forexample, the four suspensions 19, 19, . . . that secure the biaxialmovable part 15 b to the fixed part 15 a may have their spring constantschanged, while having the same diameter. Alternatively, the suspensions19, 19 may have different diameters to acquire different springconstants. More precisely, the pair of suspensions 19, 19 that lie nearthe inner circumference of the disc may have greater spring constantsthan the pair of suspensions 19, 19 that lie near the outercircumference of the disc. Otherwise, the pair of suspensions 19, 19that lie near the inner circumference of the disc may have largerdiameters larger than the suspensions 19 that lie near the outercircumference of the disc. In either way, the suspensions 19, 19 nearthe inner circumference of the disc will have greater spring constantsthan the pair of suspensions 19, 19 near the outer circumference. Thiswill make it possible to incline the objective lens 17 at a prescribedangle that is proportional to the distance δ the lens 17 has moved fromthe basic operating point A to the basic operating point B.

[0062] The objective lens 17 is displaced from the basic operating pointA, because the distance between the objective lens 17 and thesignal-recording plane of the DVD 22 a increases if the DVD 22 a iswarping or undergoes plane wobbling. If the lens 17 is so displaced, itsfocal point does not coincide with the basic operating point A. Thebiaxial actuator 15 therefore moves the objective lens 17 in thefocusing direction.

[0063] There will be explained a method in which the lens-inclinationadjusting mechanism 30 inclines the objective lens 17 at an angleoptimal to correct the inclination resulting from the warping or planewobbling of the DVD 22 a.

[0064] The distance δ of displacement and the inclination angle S havethe following relation:

S=T·δ  (1)

[0065] where T (deg/mm) is the proportionality constant.

[0066] The proportionality constant T is a constant determined by theradius R (mm) of the DVD 22 a that is a thin optical disc.

[0067] Assume that the DVD 22 a is warping is described. As FIG. 8shows, the DVD 22 a has a radius R and a warp M (mm) at its outercircumference. As seen from FIG. 8, the warp M of the DVD 22 a isuniform from the center of the DVD 22 a to the outer circumferencethereof. As already explained in connection with the conventional art,most optical discs inevitably warp and undergo plane wobbling. Thewarping or plane wobbling results in the displacement of thesignal-recording plane of an optical disc, or the inclination of theobjective lens to the optical disc. Consequently, coma-aberration willoccur, which deteriorates the beam spot that the beam forms as it isfocused on the signal-recording plane.

[0068] The lens-inclination adjusting mechanism 30, which isincorporated in the biaxial actuator 15, inclines the objective lens 17in proportion to the displacement distance δ defined by Equation 1. Themechanism 30 thus eliminates the deterioration of the beam spot, whichresults from the coma-aberration. The disc drive 1 can therefore recordand reproduce data signals on and from the DVD 22 a with highreliability.

[0069] As illustrated in FIG. 9, the inner and outer circumferences ofthe DVD 22 a are at distances R_(in) and R_(out) from the center of theDVD 22 a. The middle part of the signal-recording plane is then locatedat distance (R_(in)+R_(out))/2 from the center of the DVD 22 a. In thiscase, it is required that the proportionality constant T be of such avalue that the lens inclines at the same angle, M/R·180/π (deg), as thesignal-recording plane of the disc. If the constant T has this value, itis possible to minimize the deterioration of the beam spot, whichresults from the coma-aberration. The displacement m at this position isexpressed by the following Equation 2: $\begin{matrix}{m = {\frac{R_{i\quad n} + R_{out}}{2R} \cdot M}} & (2)\end{matrix}$

[0070] Substituting Equation 2 in Equation 1 yields the followingEquation 3 since m=δ displacement of the objective lens 17):$\begin{matrix}{{\frac{M}{R} \cdot \frac{180}{\pi}} = {T \cdot \frac{R_{i\quad n} + R_{out}}{2R} \cdot M}} & (3)\end{matrix}$

[0071] Equation 3 reduces to the following Equation 4, eliminating M andR: $\begin{matrix}{T = {{\frac{2}{R_{i\quad n} + R_{out}} \cdot \frac{180}{\pi}}\left( {\deg \text{/}{mm}} \right)}} & (4)\end{matrix}$

[0072]FIG. 10 is a diagram that shows how the angle at which theobjective lens 17 is inclined to a warping optical disc when theproportionality constant T is of the value defined by Equation 4. InFIG. 10, the position (b) is the very position that the objective lens17 should take to record or reproduce data signals on or from the middlepart of the signal-recording plane. The angle at which the lens 17should be inclined at this position (b) is equal to the inclinationangle M/R of the optical disc. The position (a) shown in FIG. 10 is onethat the objective lens 17 must assume to record or reproduce datasignals on or from the innermost part of the signal-recording plane. Theposition (c) shown in FIG. 10 is one that the objective lens 17 shouldassume to record or reproduce data signals on or from the outermost partof the signal-recording plane. Hence, the coma-aberration is reduced butinsufficiently when the objective lens 17 lies at the position (a)depicted in FIG. 10, and is reduced excessively when the objective lens17 lies at the position (c) depicted in FIG. 10. In the case of anoptical disc whose R_(in) and R_(out) are 24 (mm) and 58 (mm),respectively, T=1.397 (deg/mm). The position (b) shown in FIG. 10depicts the center of the signal-recording plane.

[0073] It is desired that the proportionality constant T should satisfythe following Equation 5, in order to record or reproduce data signalsin good conditions: $\begin{matrix}{{\frac{1}{4} \cdot \frac{1}{R} \cdot \frac{180}{\pi}} \leq T \geq {\frac{2}{R_{i\quad n} + R_{out}} \cdot \frac{180}{\pi}}} & (5)\end{matrix}$

[0074] If the proportionality constant T exceeds the upper limitspecified in Equation 5, the astigmatism will increase greater than thecoma-aberration in the course of recording or reproducing data signalson or from the outer parts of the disc, because of the extra-axialcharacteristic of the objective lens 17. This would deteriorate the beamsport. Conversely, if proportionality constant T does not reach thelower limit, the coma-aberration is reduced but insufficiently. In thiscase, too, the beam spot is deteriorated.

[0075] The objective lens 17 may be displaced from the basic operatingpoint A by the distance δ due to the warping of the DVD 22 a, which hastaken place during the manufacture thereof or due to the deformation(plane wobbling) of the mechanism that holds and rotates the DVD 22 a.In this case, the lens-inclination adjusting mechanism 30, designed bymechanically modifying some of the components, inclines the objectivelens 17 by angle S (=T·δ) that is substantially proportional to thedisplacement, while data signals are being recorded or reproduced on orfrom the DVD 22 a. The objective lens, thus inclined by angle S,prevents the beam spot from being deteriorated in spite of thecoma-aberration caused by the warping or plane wobbling of the DVD 22 a.This renders it possible to record and reproduce data signals in optimalconditions.

[0076] It will be explained how the optical pickup 12 comprising thelens-inclination adjusting mechanism 30 operates to record or reproducedata signals on or from a CD (a thick optical disc) 22 b.

[0077] To record or reproduce data signals on or from the CD 22 b byapplying a laser beam to the CD 22 b through the objective lens 17, thebiaxial actuator 15 moves the objective lens 17 from the basic operatingpoint A to the basic operating point B as illustrated in FIGS. 11A and11B. Namely, the objective lens 17 approaches the surface of the CD 22 bby a distance ΔWD from the position where it serves to record orreproduce data signals on or from the DVD 22 a, i.e., thin optical disc.

[0078] The lens-inclination adjusting mechanism 30 operates as theobjective lens 17 moves along its optical axis due to the difference inthe position of the signal-recording plane, between the DVD and the CD.Since the distance between the two basic operating points A and B is ΔWD(mm), the lens-inclination adjusting mechanism 30 inclines the objectivelens 17 by angle S=T·ΔWD (deg) obtained from Equation 1, no matterwhether the DC 22 b is inclined due to warping or plane wobbling.Generally, coma-aberration takes place when the objective lens isinclined as shown in FIG. 12A or the optical disc is inclined as shownin FIG. 12B. If the objective lens has a numerical aperture (NA) of, forexample, 0.45, coma-aberration exceeding 0.06 λrms per degree (1°) ofinclination will occur, whether the objective lens or the optical discis inclined.

[0079] Coma-aberration C_(T) that occurs when the objective lens 17 isinclined to the CD 22 b inclines the CD 22 b. This means that thecoma-aberration can be eliminated if the objective lens 17 and the CD 22b are moved to become as much parallel as is possible.

[0080] The coma-aberration C_(T), which occurs when the objective lens17 is inclined by 1° to the CD 22 b and which is indicated by brokenline in FIG. 13, can be eliminated if the CD 22 b is inclined by angle ø(deg) as seen from the solid line D shown in FIG. 13.

[0081] Hence, the coma-aberration can be eliminated at the time ofrecording or reproducing data signals on or from the CD 22 b, byinclining the CD 22 b to the objective lens 17 by the angle (deg) givenby the following Equation 6:

T·ΔWD·ø  (6)

[0082] The CD 22 b cannot be inclined to the objective lens 17 by theangle determined by Equation 6, during the process of recording orreproducing signals on or from the CD 22 b, unless the optical discdrive 1 has an additional mechanism for inclining the CD 22 b. The useof such an additional mechanism complicates the structure of the opticaldisc drive 1 and raises the manufacturing cost thereof. This contradictsthe object of the present invention.

[0083] The present invention uses no additional mechanisms and can yeteliminate the drawback resulting from the inclination of the objectivelens 17 while data signals are being recorded or reproduced on or fromthe CD 22 b, without the necessity of inclining the CD 22 b. As will bedetailed later, the light source 24 b that serves to record or reproducesignals on or from the CD 22 b is arranged off the optical axis andapplies a laser beam slantwise to the objective lens 17. Thus, theobjective lens 17 would not adversely influence the beam spot when it isinclined at the basic operating point B. The method of nullifying theinfluence that the objective lens 17 inclined by angle T·ΔWD at thebasic operating point B imposes on the beam spot will be described belowin detail.

[0084] The second light source 24 b of the laser element 24, which isused for the CD 22 b, is located at a distance ΔL (mm) from the firstlight source 24 a used for the DVD 22 a lying on the optical axis and atan angular distance θ (measured counterclockwise from the intersectionof the radius and the outer circumference). The laser beam applied tothe CD 22 b to record or reproduce data signals is inclined by angle γ,which is determined by the following Equation 7; $\begin{matrix}{{\tan \quad \gamma} = \frac{\Delta \quad L}{fCL}} & (7)\end{matrix}$

[0085] where fCL (mm) is the focal distance of the collimator lens 27.

[0086] In Equation 7, the beam inclination γ can be decomposed into aradial part and a vertical (tangent) part, which are represented by γcos θ and γ sin θ, respectively.

[0087] In the process of recording or reproducing data signals on orfrom the CD 22 b by applying a laser beam via the objective lens 17, asshown in FIGS. 15A and 15B, coma-aberration Cy occurs when the beamapplied from the light source 24 b is inclined to the objective lens 17by the unit angle. To eliminate this coma-aberration Cy, the CD 22 bneeds to be inclined at a specific angle α (deg) to record or reproducesignals in optimal conditions. (The direction in which the CD 22 bshould be inclined at the angle α is regarded as positive.) When theobjective lens 17 lies perpendicular to the optical axis, data signalswill be recorded or reproduced on or form the CD 22 b most reliably ifthe CD 22 b is inclined in the directions given by the followingEquation 8:

(a)Radial direction:αγ cos θ

(b)Tangential direction:αγ sin θ  (8)

[0088] From Equations 6 and 8, it is clear that data signals can berecorded or reproduced on or from the CD 22 b in the most desirable wayif the CD 22 b is inclined to both the radial direction and thetangential direction by angles specified by the following Equation 9:

(a)Radial direction:ΔTilt(rad)=T·ΔWD·ø+γα cos θ

(b)Tangential direction:ΔTilt(tan)=γα sin θ  (9)

[0089] In most optical pickups that can record and reproduce datasignals on and from various types of optical discs, however, thecomponents for supporting and rotating an optical disc (e.g., disc table11 and the like) need to be made of the same material to reduce themanufacturing cost and to simplify the structure. In view of this, it ispractically impossible to incline the CD 22 b. Thus, measures are takenin this invention to both values (a) and (b) in Equation 9 are reducedto zero, thereby preventing the deterioration of the beam spot. Thisrenders it possible to record and reproduce data signals with highreliability.

[0090] In practice, it suffices to give a negative value to cos θ, i.e.,the second term of (a) of Equation 9, since T, ΔWD, ø, γ, and α areusually positive values. Considering (b) of Equation 9, it is desiredthat value should fall within the range of 120°≦θ≦240°.

[0091] Considering the errors in manufacture, the tolerance for theinclination angle should be ±0.2° or less for the tangential directionand ±0.3° or less for the radial direction. If this requirement isachieved, the deterioration of data signals recorded or reproduced willfall within the tolerable margin. Thus, it is sufficient to satisfy thefollowing Equation 10:

(a)|ΔTilt(rad)|≦0.3

(b)|ΔTilt(tan)|≦0.2  (10)

[0092] In the optical disc drive 1 incorporating the optical pickup 12,data signals can be reliably recorded and reproduced on and from the CD22 b even when the lens-inclination adjusting mechanism 30 inclines theobjective lens 17 at the operating point B as shown in FIGS. 11A and11B, only if the following Equation 11 is satisfied. $\begin{matrix}{{{(a) - 0.3} \leq {{{T \cdot \Delta}\quad {{WD} \cdot \varphi}} + {{{\tan^{- 1}\left( \frac{\Delta \quad {L \cdot {\beta }}}{f} \right)} \cdot \alpha \cdot \cos}\quad \theta}} \leq 0.3}{{(b) - 0.2} \leq {{{\tan^{- 1}\left( \frac{\Delta \quad {L \cdot {\beta }}}{f} \right)} \cdot \alpha \cdot \sin}\quad \theta} \leq 0.2}} & (11)\end{matrix}$

[0093] where β is the imaging magnification of the optical system 23 andf is the focal distance of the objective lens 17.

[0094] When signals are recorded or reproduced on or from the CD 22 b,the light source 24 b of the laser element 24, which is provided forCDs, is located at distance ΔL from the light source 24 b provided forDVDs and inclined at angle θ. Then, the laser beam emitted from thelight source 24 b applied slantwise to the objective lens 17, therebyeliminating the coma-aberration caused because the lens-inclinationadjusting mechanism 30 has inclined the objective lens 17 at angleS(=T·ΔWD). Now that the coma-aberration has been eliminated, the beamspot is not deteriorated at all. Hence, data signals can be recorded andreproduced in good conditions.

[0095] As has been described, the present invention provides an opticalpickup 12 and an optical disc drive 1 which can record and reproducedata signals from optical discs (e.g., a CD and a DVD) of differentspecifications, by using one and the same objective lens. In the opticalpickup 12 and the optical disc drive 1, a lens-inclination adjustingmechanism 30 is provided. The mechanism 30 is designed by modifyingmechanical features, without using additional components. The mechanism30 inclines the objective lens 17, eliminating the deterioration of thebeam spot, caused by the warping of the disc, the deformation of thedisc-rotating mechanism or the plane wobbling occurring while the discis being rotated. A laser beam is applied slantwise to the CD 22 b bymoving the light source 24 b from the optical axis, in spite of the factthat the lens-inclination adjusting mechanism 30 has inclined theobjective lens 17 because the objective lens 17 is located closer to thesignal-recording plane of the CD 22 b than to that of the DVD 22 a.Thus, a beam spot not deteriorated at all can be formed on both types ofoptical discs. Therefore, data signals can be recorded and reproduced onand from both types of optical discs, in optimal conditions.

[0096] The shape and structure of any component of the embodimentdescribed above are no more than examples. They should not beinterpreted to limit the present invention.

INDUSTRIAL APPLICABILITY

[0097] As described above, the optical pickup and an optical disc drivecomprising this pickup can prevent deterioration of the beam spot andcan therefore record and reproduce data signals, in optimal conditions,on two types of optical discs that differ in specifications, such asrecording density and thickness. In particular, the deterioration of thebeam spot formed on the disc of one type, caused by the warping of thedisc, is eliminated to record or reproduce data signals on or from theoptical disc. Further, data signals can be recorded and reproduced onand from the disc of the other type even if the beam spot formed on thedisc is deformed since a lens-inclination adjusting mechanism inclinesthe object lens. Thus, it is possible to record and reproduce the datasignals on and from either type of an optical disc in good conditions.

1. An optical pickup designed to record or reproduce data signals on orfrom two types of optical discs that differ in at least data recordingdensity and thickness, by using the same objective lens, said opticalpickup comprising: two light source sections for emitting laser beamshaving different wavelengths; and a lens-supporting mechanism forcontrolling a position of the objective lens in a radial direction ofthe optical discs and along an optical axis, wherein the lens-supportingmechanism has a lens-inclination adjusting mechanism capable of changingan angle at which the objective lens is inclined to the optical axis,substantially in proportion to a displacement of the optical disc of thefirst type, and the second light source section for recording orreproducing data signals on or from the optical disc of the second typeis positioned with respect to the position of the first light sourcesection for recording or reproducing data signals on or from the opticaldisc of the first type, so as to satisfy the following condition:120°≦θ≦240° where θ is an angle measured counterclockwise from the firstlight source section used to record or reproduce data signals on or fromthe optical disc of the first type to the second light source sectionused to record or reproduce data signals on or from the optical disc ofthe second type, along a straight line in which the optical pickup movestoward an outer circumference of the optical disc to record or reproducedata signals on or form the optical disc.
 2. The optical pickupaccording to claim 1, wherein the two light source sections aresupported on the same support member.
 3. The optical pickup according toclaim 1, wherein the lens-inclination adjusting mechanism is configuredto change the angle at which the objective lens is inclined, to satisfythe following condition: 1/4·δ/R≦S≦δ/R where S is the inclination angleof the objective lens, δ is the displacement of the objective lens froma reference position, and R is the radius of the optical disc.
 4. Theoptical pickup according to claim 1, further comprising alight-receiving section for receiving laser beams reflected by theoptical discs, and means arranged between the light-receiving sectionand the objective lens, for combining optical paths of two types oflaser beams emitted from the two light sources, respectively, such thatthe two laser beams reflected by the optical discs are received atsubstantially the same point on the light-receiving section.
 5. Theoptical pickup according to claim 1, which satisfies the followingconditions: −0.3≦T·ΔWD·ø+tan⁻¹(ΔL·|β|/f)·α·cos θ≦0.3−0.2≦tan⁻¹(ΔL·|β|/f)·α·sin θ≦0.2 where f is the focal distance of theobjective lens; β is the magnification of the optical system; ΔWD is thedifference between the working distance of recording or reproducing datasignals on or from the optical disc of the first type and the workingdistance of recording or reproducing data signals on or from the opticaldisc of the second type; ø is the angle at which the optical disc of thesecond type is inclined to record or reproduce the signals in optimalconditions when the incidence angle of the laser beam applied to theobjective lens changes by one degree; α is the angle at which theoptical disc of the second type is inclined to optimize the signalsrecorded or reproduced, when the incidence angle of the laser beamapplied to the objective lens changes by the one degree (regarded aspositive if the disc is inclined to eliminate the influence ofinclination of the laser beam); T is the proportionality constantdetermined by the radius of the optical disc of the first type, whichrelates the displacement of the objective lens (regarded as positive ifthe objective lens approaches the optical disc of the first type) from areference position and the inclination of the objective lens (regardedas positive if the side of the objective lens located at the outercircumference of the optical disc of the first type approaches theoptical disc of the first type); ΔL is the distance between the firstlight source section used to record or reproduce data signals on or fromthe optical disc of the first type and the second light source sectionused to record or reproduce data signals on or from the optical disc ofthe second type; and θ is an angle measured counterclockwise from thefirst light source section to the second light source section, along astraight line in which the optical pickup moves toward an outercircumference of the optical disc to record or reproduce data signals onor from the optical disc.
 6. An optical disc drive apparatus designed torecord or reproduce data signals on or from two types of optical discsthat differ in specifications such as data recording density, thicknessand the like, by using the same objective lens, said optical disc drivecomprising: an optical pickup comprising two light source sections foremitting laser beams having different wavelengths, and a lens-supportingmechanism for controlling a position of the objective lens in a radialdirection of the optical discs and along an optical axis, wherein thelens-supporting mechanism has a lens-inclination adjusting mechanismcapable of changing an angle at which the objective lens is inclined tothe optical axis, substantially in proportion to a displacement of theoptical disc of the first type from a reference point in asignal-recording plane of the optical disc of the first type, and thesecond light source section for recording or reproducing data signals onor from the optical disc of the second type is positioned with respectto the position of the first light source section for recording orreproducing data signals on or from the optical disc of the first type,so as to satisfy the following condition: 120°≦θ≦240° where θ is anangle measured counterclockwise from the first light source section usedto record or reproduce data signals on or from the optical disc of thefirst type to the second light source section used to record orreproduce data signals on or from the optical disc of the second type,along a straight line in which the optical pickup moves toward an outercircumference of the optical disc to record or reproduce data signals onor from the optical disc.
 7. The optical disc drive apparatus accordingto claim 6, wherein the two light source sections are supported on thesame support member.
 8. The optical disc drive apparatus according toclaim 6, wherein the lens-inclination adjusting mechanism is configuredto change the angle at which the objective lens is inclined, to satisfythe following condition: 1/4·δ/R≦S≦δ/R where S is the inclination angleof the objective lens, δ is the displacement of the objective lens froma reference position, and R is the radius of the optical disc.
 9. Theoptical disc drive according to claim 6, further comprising alight-receiving section for receiving laser beams reflected by theoptical discs, and means arranged between the light-receiving sectionand the objective lens, for combining optical paths of two types oflaser beams emitted from the two light sources, respectively, such thatthe two laser beams reflected by the optical discs are received atsubstantially the same point on the light-receiving section.
 10. Theoptical disc drive according to claim 6, which satisfies the followingconditions: −0.3≦T·ΔWD·ø+tan ⁻¹(ΔL·|β|/f)·α·cos θ≦0.3−0.2≦tan⁻¹(ΔL·|β|/f)·α·sin θ≦0.2 where f is the focal distance of theobjective lens; β is the magnification of the optical system; ΔWD is thedifference between the working distance of recording or reproducing datasignals on or from the optical disc of the first type and the workingdistance of recording or reproducing data signals on or from the opticaldisc of the second type; ø is the angle at which the optical disc of thesecond type is inclined to record or reproduce the signals in optimalconditions when the incidence angle of the laser beam applied to theobjective lens changes by one degree; α is the angle at which theoptical disc of the second type is inclined to optimize the signalsrecorded or reproduced, when the incidence angle of the laser beamapplied to the objective lens changes by the one degree (regarded aspositive if the disc is inclined to eliminate the influence ofinclination of the laser beam); T is the proportionality constantdetermined by the radius of the optical disc of the first type, whichrelates the displacement of the objective lens (regarded as positive ifthe objective lens approaches the optical disc of the first type) from areference position and the inclination of the objective lens (regardedas positive if the side of the objective lens located at the outercircumference of the optical disc of the first type approaches theoptical disc of the first type); ΔL is the distance between the firstlight source section used to record or reproduce data signals on or fromthe optical disc of the first type and the second light source sectionused to record or reproduce data signals on or from the optical disc ofthe second type; and θ is an angle measured counterclockwise from thefirst light source section to the second light source section, along astraight line in which the optical pickup moves toward an outercircumference of the optical disc to record or reproduce data signals onor from the optical disc.
 11. An optical pickup comprising: two lightsource sections for emitting laser beams having different wavelengths; alens-supporting mechanism for controlling a position of the objectivelens in a radial direction of the optical discs and along an opticalaxis, wherein the lens-supporting mechanism has a lens-inclinationadjusting mechanism capable of changing an angle at which the objectivelens is inclined to the optical axis, substantially in proportion to adisplacement of the optical disc of the first type from a referencepoint in a signal-recording plane of the optical disc of the first type,and the lens-inclination adjusting mechanism is configured to change theangle at which the objective lens is inclined, to satisfy the followingcondition: 1/4·δ/R≦S≦δ/R where S is the inclination angle of theobjective lens (regarded as positive if the objective lens approachesthe outer circumference of the optical disc), δ is the displacement ofthe objective lens from a reference position (regarded as positive ifthe objective lens approaches the optical disc), and R is the radius ofthe optical disc.